The use of a hydrodynamic step in a water hull for aircraft use is attributed by some to Glenn Curtiss. Early floatplanes designed by Curtiss had difficulty in becoming airborne, regardless of the amount of power applied or wing surface provided. It was discovered that the float or hull of the craft would tend to "stick" to the water surface due to the laminar flow of the water.
The simple solution was to introduce a drag-inducing (and vortex generating) step at the rearward portion of hull or float to reduce or eliminate water adhesion and allow the craft to separate from the water flow. Unfortunately, this same step design also generates air vortices once the craft is airborne, thus contributing to aerodynamic drag.
Early aircraft utilized construction techniques resulting in high drag coefficients. Struts, wires, and braces in the airstream resulting in high drag designs. Thus, the amount of drag introduced by a hydrodynamic step contributed relatively little to the overall drag of the craft. However, modern aircraft are much more carefully designed to eliminate drag and have highly efficient aerodynamic designs. In such a design, a float or hull step may comprise a significant source of drag.
Curtiss U.S. Pat. No. 1,420,609, issued Jun. 20, 1922, discloses an early amphibious airplane. This reference is one of a number of Curtiss patents which represent the earliest float planes in the art. Given the high drag losses of these early planes, the concept of retractable step (or indeed the concept of a step at all) was not known. However, the idea of retractable wingtip floats appears to have been disclosed in at least one Curtiss patent.
Martin U.S. Pat. No. 1,728,621, issued Sep. 17, 1929, discloses a seaplane with retractable float. In the Martin design, the entire float retracts into the fuselage of the airplane to improve aerodynamics. However, the step portion of the airplane still remains in the airstream.
Von Miller U.S. Pat. No. 1,794,474, issued Mar. 3, 1931, discloses a pontoon with retractable doors. Von Miller takes an unusual approach to reducing drag with pontoons by providing front and rear doors on the pontoon. In flight, the doors are opened allowing air to pass through the center portion of the pontoon. In addition to sealing problems, such a design might not significantly reduce drag, but actually increase it. The pontoons, with doors open, act as a flow orifice, which in and of itself creates drag.
Weihmiller U.S. Pat. No. 2,214,945, issued Sep. 17, 1940, discloses a retractable hydrovane. The hydrovane appears to be of the hydrofoil variety in that it allows the boat to plane on the V-shaped hydrofoil. Note, however, how his hydroplane retracts into the step portion (FIG. 4) forming an aerodynamic fairing.
Von Schlippe U.S. Pat. No. 2,228,860, issued Jan. 14, 1941, discloses an apparatus allowing plane to land in the water under emergency conditions. Von Schlippe provides an aerodynamic flotation device which may be used by an airplane for emergency landings. To make the float more aerodynamic, Von Schlippe puts a fairing over the step portion. The fairing is made of a water soluble material so that upon landing, the fairing will dissolve in water exposing the step. This reference does disclose the idea of placing a fairing over a step to enhance streamlining. However, the Von Schlippe device only has a single use application.
Stampfl U.S. Pat. No. 2,265,206, issued Dec. 9, 1941, discloses a hydroplane boat hull. This reference is relevant to the extent that it discloses an extendable planing board which may extend into the water stream. An inflatable bladder is used to extend the planing board and spring action is used to retract the planing board. This reference is relevant to the extent that it discloses the use of movable surfaces in a hydroplane boat hull. However, in Stampfl, the planes extend into the waterstream. Moreover, Stampfl appears to be directed toward a boat and not an airplane, and thus the motivation to apply aerodynamic techniques to Stampfl do not exist.
Dornier U.S. Pat. No. 2,320,574, issued Jun. 1, 1943, discloses a retractable float system for a seaplane. FIG. 5 is of particular interest. The entire boat shaped section of the hull is covered over with a retractable float which may be retracted to form a smooth fuselage. When landing, the floats are extended, exposing the boat shaped fuselage and adding additional flotation to the side of the aircraft.
Parker U.S. Pat. No. 2,347,841, issued May 2, 1944, discloses a seaplane having extendable step portions. In flight, the portions are retracted to form a smooth surface. For take off and landings, primary and secondary steps 9, 10, and 11 are extended using a hydraulic system. However, this system actually discloses an extendable step. Also note the large surface areas employed by Parker and the very complicated hydraulic system used.
Bloomfield et al. U.S. Pat. No. 2,958,486, issued Nov. 1, 1960, discloses a retractable planing device for a float plane. This device is not a step per se. This device extends to form a planing board and retracts in flight.
Arney et al. U.S. Pat. No. 5,692,706, issued Dec. 2, 1997, discloses an inflatable float step reinforcement system. Arney is directed toward inflatable floats, such as the FULL LOTUS.TM. floats used in ultralights and small planes. An inflatable bladder is provided at the step portion of the float. Pressure within the bladder may be regulated to alter the shape and size of the step.
Although a number of prior art references appear to start to address the idea of reducing drag formed from an aerodynamic step, none of the devices appear to have been successful in the marketplace, most likely due to the increased weight and cost of such devices. What remains a requirement in the art is a hydrodynamic step which can be faired once in flight so as to reduce aerodynamic drag without the need for a complex, heavy, and expensive mechanism.